1. Field of the Invention
The invention relates to the preparation of citric acid and salts of citric acid.
2. Description of the Prior Art
Citric acid or its salts are useful in different ways as exemplified by the following patents: As a plasticizer, U.S. Pat. No. 2,409,703; as a bleaching agent, U.S. Pat. No. 2,529,831; as a food antioxidant, U.S. Pat. No. 2,563,855; as a detergent component, U.S. Pat. No. 2,765,280.
The principal prior sources of citric acid and its derivatives are recovery from natural products such as citrus fruits, and production via micological or fermentation processes. The recovery of citric acid from natural products or sources is disclosed in U.S. Pat. Nos. 2,027,264; 2,193,904; and 2,396,115. The production of citric acid by micological processes is disclosed in U.S. Pat. Nos. 2,353,771; 2,739,923; 2,883,329; and 3,335,067.
Heretofore, the chemical synthesis of citric acid and its salts has proved to be very difficult. In fact, the only known U.S. Pat. No. relating to a chemical synthesis of citric acid is 3,356,721 which issued in 1967 and there is nothing in the patent to show that a significant yield of citric acid or its salts is obtained with the process described therein. Since the amount of natural source citrate is limited, there has been a need in the art for a commercially feasible chemical synthesis process for producing citric acid or its salts.
It is an object of the present invention to provide a process for synthesizing citric acid and salts of citric acid from readily available moderate cost raw materials.
Another object of the present invention is to provide process operations for producing new compositions which are useful intermediates for the synthesis of citric acid and salts of citric acid.
Another object of the present invention is to provide a process for producing intermediate compositions that can be hydrolyzed to produce citric acid or its salts in high yield.
Another object of the present invention is to provide a process for producing intermediate compositions of the preceding objects via a reaction of 3-carbamoyl-3-hydroxy-4-halobutyric acid with cyanide and with alkaline earth metal hydroxide in an aqueous system.
Another object of the present invention is to provide a process for producing 3-carbamoyl-3-hydroxy-4-chlorobutyric acid and for converting that acid to another intermediate useful in the production of citrate wherein it is unnecessary to purify the 3-carbamoyl-3-hydroxy-4-chlorobutyric acid to remove excess cyanide and other impurities associated with the production thereof prior to the conversion.
Another object of the present invention is to provide a process for producing soluble or insoluble alkaline earth metal adduct intermediates which are hydrolyzable to citrate by reaction with acid or base and which process provides inherent self-control of pH in a preferred range for optimum conversion.